WWC - Condensation and Precipitation Lesson

 

Condensation and Precipitation

Since water molecules evaporate from a liquid individually, you cannot see water vapor. There are individual water molecules in the air surrounding you, even though you cannot see them; the measure of water vapor in the air is known as humidity. However, individual water molecules may eventually lose enough energy to reform hydrogen bonds with other molecules, clinging together to form larger groups.

When you see steam rising above boiling water, you are actually seeing very tiny clumps of water molecules clinging together. The process of individual water molecules clinging together is called condensation, and usually involves water molecules interacting with another substance. For example, individual water molecules may cling to a blade of grass early in the morning, forming dew. Or, they may cling to airborne particles such as dust or salt to form clouds (high in the air) or fog (close to Earth's surface). Airborne water molecules will cling to the outside of an ice-cold drinking glass, making the glass seem to "sweat", and leaving water rings on furniture.

The formation of condensation in the air leads to another phenomenon. Clouds consist of countless tiny droplets of water, and each droplet continually changes in size, depending on atmospheric conditions. Additional water molecules are added to the droplets through condensation, while other water molecules are evaporated out of the droplet. As long as atmospheric forces - thermal masses and wind - keep the droplets in the air, the cloud will remain aloft. If the droplets gain enough mass, however, they fall back to the Earth's surface as precipitation.

Keeping in mind that precipitation is condensed water that became heavy enough to fall to the Earth's surface, and that condensation occurs when water molecules bond to "seed particles" (such as airborne dust, salt, smoke, etc.), precipitation isn't actually pure water. However, the contamination in it is very slight, so the water that falls to the Earth is still considered "fresh". Precipitation is by far the most common source of freshwater on Earth although it is not evenly distributed. The average amount of precipitation on Earth is 39 inches, but much of that rainfall (approximately 80%) occurs over the oceans.

Precipitation can occur in several different forms. Rain, the liquid form of precipitation, is the most common, and liquid rain that comes into contact with a below-freezing environment is known as freezing rain. Precipitation may also occur as snow, sleet, or hail. Snow forms when small condensation droplets freeze, and additional water molecules condense on them, forming crystalline structures. Sleet occurs when falling snow passes through a warm layer of air and then into another cold layer of air. The warm layer melts the snow, and the cold layer freezes the droplets into tiny ice pellets that fall to the Earth. Hail begins in a fashion similar to snow and sleet, but storm conditions continue to keep the pellets aloft, allowing more and more accumulation of condensation around the pellet until the weight of the pellet is enough to overcome the updraft of the storm. Hail is generally 5mm or more in diameter, but can occasionally grow several centimeters or more in diameter.

 

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